Since I'm not sure if this is a well - known novel, I can't really say what it's about. It could potentially be about a character who borrows something and then a spider comes into the story in some significant way, perhaps as a symbol or an actual part of the plot.
Power struggles are also quite common. There are usually other mages or magical beings that the spider mage has to contend with for power or resources. For example, in some fics, there are rival mages who want to steal the spider mage's unique powers.
I'm not sure specifically as I haven't read that particular fanfic. It could be about a spider - perhaps a spooky or mysterious one that arrives at midnight. Maybe it has elements of mystery, danger or a unique story related to the spider's appearance at that time.
I'm not sure there is a widely known novel exactly named 'Borrow Along Came a Spider'. It might be a very specific or self - published one. You could try searching for it in specialized indie bookstores or online platforms that deal with less mainstream literature.
It could start with a strange occurrence where spider - like creatures start appearing all over the city. Supergirl and Donna begin to investigate. As they dig deeper, they find out that there is a powerful force behind it all. They face many challenges along the way, like dealing with the webs that the spider - creatures create which can trap even Supergirl. Donna might use her combat skills to fight the smaller creatures while Supergirl takes on the bigger threats. And in the end, they discover the mastermind behind it all and save the city.
You could also set the fan fiction in a different time period. For example, if the original was set in modern times, you could move it to the past, say the Victorian era, and show how the spider's presence might have been perceived differently. Or you could take it to a post - apocalyptic future where the spider has somehow survived and adapted in a strange new world.
The plot might center around a web - like conspiracy (a nod to the 'Along Came a Spider' theme). Kara could be the muscle, Peter the brains and Donna the heart of the group. They start investigating something small but soon find out it's part of a much larger and more dangerous scheme.